The present invention relates to a battery management system for an electric vehicle, and more specifically to battery management system for monitoring the status such as a remaining battery capacity of a high voltage battery mounted on the electric vehicle by detecting various battery conditions, for example, terminal voltage, temperature, etc.
Recently, an electric vehicle has been developed more and more. In the electric vehicle, it is important to always detect the status of the battery mounted thereon, so that various battery management system have been so far proposed.
For instance, Japanese patent application laid-open (Kokai) No. 5-227603 (1993) discloses such a battery management system that: various drive data related to an electric vehicle (e.g., vehicle starting and traveling conditions) detected by various vehicle sensors and various data related to battery charged status such as charge time, charged capacity or the number of charged times, which is detected by various battery sensors are recorded in an IC (integrated circuit) card; and that the remaining battery capacity is read and displayed for the driver where necessary, so that the car battery can be charged timely and effectively.
Further, Japanese patent application laid-open (Kokai) No. 6-54402 (1994) discloses a battery management system such that: the remaining capacity of the battery can be obtained accurately, with the use of a bi-directional power capacity meter for detecting a power capacity of the battery mounted on an electric vehicle; reading means for reading battery-related data such as specific battery data, history data, from a recording medium as battery power correction data for correcting the battery power capacity; and calculating means for calculating a corrected battery power capacity on the basis of the power capacity detected by the bi-directional power capacity meter and the correction data obtained by the reading means.
In the above-mentioned prior art battery management system, however, it is necessary to always obtain the correction value of the remaining capacity of the vehicle drive battery, by detecting the battery terminal voltage, the battery temperature and the time elapsed after released open from discharge, not only during traveling but also when the key switch is kept turned off (i.e., when, the electric vehicle is kept stopped or parked at night or for many hours in daytime).
In this method, however, since the current consumed for a battery management system is relatively large, when the battery management system is kept activated, even after the key switch has been turned off such as during a parking, there exists a possibility that the car battery is used up.
Here, the reason why the battery must be monitored even after the battery is released open from discharge, will be explained hereinbelow with reference to FIGS. 1(a) to 1(e)
As shown in FIG. 1(a), when the electric vehicle is stopped and further the key switch is turned off at t.sub.0 ; that is, the terminal battery voltage is restored relatively sharply up to a peak value after about 30 minutes at t.sub.1 when the battery is released open. After that, the terminal voltage begins to decrease gradually. Here, the time between t.sub.0 and t.sub.1 is referred to as a specific time T.sub.p. FIG. 1(b) shows this restoration status in an enlarged scale, which indicates that it is possible to know the deterioration status of the drive battery by measuring the restoration rate (.DELTA.V/.DELTA.t) of the terminal voltage with the lapse of time. In other words, a criterion for exchange the battery with new one can be known.
However, this restoration rate of the battery obtained immediately after the battery has been released open differs according to the past discharge status of the battery. FIG. 1(c) shows an example where a small power is discharged for a long time; FIG. 1(d) shows another example where a large power is discharged for a short time; and FIG. 1(e) shows the other example where a small power is discharged for a medium time.
Therefore, when the battery is always monitored so often, even after the key switch has been turned off in the same way as with the case where the key switch is turned on, there exists a problem in that the current consumption required for the battery management system increases, with the result that the battery is used up for battery management or monitoring.